Switching arrangements are key components of transceiver systems for use in e.g. microwave applications or other high frequency applications. A common application of such switching arrangements is a transmit/receive switch in wireless systems, which may be used for switching a connection of an antenna to a low noise amplifier (LNA) in a receiving mode, and a connection of the antenna to a power amplifier (PA) in a transmitting mode. Such a switch is called single-pole double-throw (SPDT) switch.
Other applications may require a switch with two inputs and two outputs, which in this case is called a double-pole double-throw (DPDT) switch. For example, in consumer satellite communication systems, two polarities can be received at a receiver input, i.e. a vertical and a horizontal polarity. In order to provide a watch-and-record capability of a satellite receiver, two down converters are required, one for providing the polarization and band selection for the watch functionality, and a second one for selecting the polarization and band selection for the recording functionality. The switching functionality can be provided by using a DPDT switch as an interface between the two antennas and the two down-converters.
The DPDT switch is commonly also known as transfer switch and may be available in UHF/VHF (Ultra High Frequency/Very High Frequency) bands covering 10 MHz to 2 GHz, and microwave bands covering 10 MHz to 20 GHz and above.
FIG. 1 shows four possible switching states A to D of a DPDT switch and a block diagram of a possible implementation. It is noted that the switching states C and D switch a single input to two outputs, while the second input is left unconnected. The block diagram on the right side of FIG. 1 shows a DPDT switch 10 as a parallel combination of SPDT switches 11, 12. The switching states A to D can be selected based on control voltages supplied to corresponding control terminals of the DPDT switch.
Many microwave switches use PIN diodes in their implementation. A forward biased PIN diode can be regarded as a current controlled resistor wherein the resistance decreases with increasing forward current due to increased carrier density in the intrinsic zone (I). A reverse biased PIN diode can be regarded as a voltage-controlled capacitor, wherein the capacitance decreases with increasing reverse voltage due to an increased width of the intrinsic zone (I).
R. Tayrani et al., “Broad-Band SiGe MMICs for Phased-Array Radar Applications”, IEEE JOURNAL OF SOLID-STATE CIRCUITS, 38 (9): 1462-1470, September 2003, describes use of standard diodes (PN diodes without intrinsic zone) available in mainstream IC (Integrated Circuit) technologies, instead of PIN diodes. In particular, a SPDT switch with PN diodes is described there.
FIG. 2 shows a schematic circuit diagram of the SPDT switch as described in the above prior art, wherein two branches connected to a second port P2 and a third port P3 each have a series diodes D1, D3 and a shunt diodes D2, D4 in each path. Such diode structures consisting of a series diode and a shunt diode are called series-shunt diode structures. If a first port P1 is connected to the second port P2, then the series diode D1 is forward biased with a given value of DC current, and the shunt diode D2 is in reverse bias which creates a low parasitic capacitance in parallel to the signal path. At the same time, the series diode D3 in the branch of the third port P3 is set in reverse bias, and the shunt diode D4 is biased with a DC current, which effectively short-circuits the third port P3 to ground. This arrangement improves isolation of the switch in comparison to just a single series diode in the signal path. The biasing of the diodes is achieved by applying biasing voltages Vb1 to Vb3.
It is noted that the circuit diagram in FIG. 2 is based on a microstrip or stripline technology with microstrip or stripline portions indicated as rectangular blocks or t-shaped blocks. Resistors R1 to R3 are indicated in FIG. 2 as triangular wave patterns. In contrast thereto, resistors are indicated in the following figures as rectangular blocks.
Thus, PN diodes are connected to the transmission line in series or in shunt. Isolation is achieved by reverse biasing series-connected diodes or forward biasing shunt-connected diodes. The shunt-connected diode provides the most effective means for achieving broadband and relatively frequency independent isolation. It is ideally frequency independent, but practically small parasitic reactances generally effect broadband performance. Isolation is also achieved by reverse biasing series-connected diodes. Isolation for series-connected diodes decreases with increasing frequency. The combined series-shunt diode configurations or structures are frequently employed in multi-throw broadband switches to achieve relatively high isolation in a simple structure.
However, the above conventional switching arrangement is disadvantageous in that it needs control voltages of 0V, 3.8V and a negative value of −6.8V. Furthermore, the achieved isolation might not be good enough for specific applications, and a relatively high current consumption of e.g. 2 mA is observed.
In addition, a further drawback resides from the fact that the SPDT switch of FIG. 2 cannot be used as a modular switch for building higher order switches, such as a DPDT switch shown in FIG. 1. The reason therefore is that an unused input in the SPDT switch of FIG. 2 is shunt or short-circuited to ground to improve isolation as discussed above. This is however not compatible with the above switching states A and B in FIG. 1.
It is therefore an object of the present invention to provide an improved switching arrangement with a modular switching cell structure which can be used to built SPDT, DPDT and higher order switches.